1. Field of the Invention
This invention relates generally to a zoom lens apparatus, and more particularly to a zoom lens apparatus with a variable frame lens group, an aberration correcting lens group and a focus correcting lens group.
2. Description of Related Art
In the past, a zoom lens apparatus performed the zooming by using a variable frame lens group and a focus correcting lens group. Recently, these two lens groups and an aberration correcting lens group have been used to improve the zooming performance.
In such the conventional zoom lens apparatus, a zoom cam cylinder is rotatably arranged in a lens barrel, and three kinds of cam grooves are formed on the zoom cam cylinder. Cam followers of the variable frame lens group, cam followers of the aberration correcting lens and cam followers of the focus correcting lens group are fitted in the three kinds of the cam grooves, respectively. The cam followers are arranged through the cam grooves to be fitted in straight grooves of a fixed cylinder, which is arranged outside the zoom cam cylinder. Thus, rotating the zoom cam cylinder moves the variable lens group, the aberration correcting lens group and the focus correcting lens group forward and backward along an optical axis due to the operation of the cam followers, the cam grooves and the straight grooves.
In the conventional zoom lens apparatus, however, the three kinds of cam grooves are formed on the zoom cam cylinder in order to move the variable frame lens group, the aberration correcting lens group and the focus correcting lens group, respectively. Consequently, the zoom cam cylinder must be long, and the zoom lens apparatus must be large.
To address this problem, the applicant of the present invention has already proposed a zoom lens apparatus, in which one cam groove is commonly used for two lens groups among the variable frame lens group, the aberration correcting lens group and the focus correcting lens group (U.S. Pat. No. 5,907,439 corresponding to Japanese Patent provisional publication No. 10-142471). A cam follower of one lens group of the two lens groups is pressed against one side wall of the cam groove, and a cam follower of the other lens group is pressed against the other side wall of the cam groove.
This zoom lens apparatus, however, uses one coil spring with a large diameter in order to press the cam followers of the two lens groups against the side walls of the cam groove, and the coil spring is arranged between the two lens groups. For this reason, a plurality of cam followers (e.g., three) provided at each lens supporting frame for each lens group are not pressed uniformly. This results in deviation of the optical axis of the lens group at the zooming when the lens supporting frame comes in contact with a stopper at the zoom end. Consequently, image on a monitor gets rough.